Modeling agencies are companies that represent models. They find them work and negotiate their contracts.
Modeling agencies will also help models with their portfolios as well as manage their career. After a model is signed, he or she will be assigned an agent; generally, this representative will collect ten percent of the earnings models receive on jobs. To find representation, you can send a headshot and resume to the agency by mail. If they feel you are the type of model they want in their stable, they will call you in for an interview and perhaps take some test photos of you.
More information can be found by visiting models agencies.